The present invention relates to a semiconductor laser and a method of fabricating the same. More particularly, the present invention relates to a semiconductor laser using an InGaAlBN based material and a method of fabricating the same.
Recently, development of a semiconductor laser using an InGaAlBN based material has been driven as a short wavelength light source, which is required for an optical disc with a higher recording density or the like. A semiconductor laser made of this kind of material can emit a beam having a small diameter in its adaptation to a short wavelength and therefore it is hoped that the laser is put into practice as a light source for high-density information processing such as an optical disc. A semiconductor laser using a multi-quantum-well-structure, as a structure realizing oscillation by current injection in this material system, has been reported, for example, in the following articles:
1) S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku and Y. Sugimoto: "InGaN-based multi-quantum-well-structure laser diodes", Jpn. J. Appl. Phys., 35 (1996) pp. L74-L76.
2) S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku and Y. Sugimoto: "InGaN multi-quantum-well-structure laser diodes with cleaved mirror facets", Jpn. J. Appl. Phys., 35 (1996) pp. L217-L220.
It is known that a multi-quantum-well-structure using a thin film active layer can reduce a threshold value much, as compared with the case of a bulk active layer. In an InGaAlN based material, however, a threshold current density is still high and an operating voltage are also high and, therefore, there remain many problems to realize a continuous oscillation.
One of causes by which the operating voltage in the InGaAlN based material is high is that a contact resistance in the case of p-type is extremely high. In a stripe geometry of an electrode, which has been already reported, a voltage drop in a p-type electrode stripe is large and not only the operating voltage becomes high but also heat generation in the region cannot be neglected. In order to reduce a contact resistance, it is simply considered to expand an electrode area, but in such a broad stripe geometry a magnitude of a threshold current becomes larger and a fundamental transverse-mode oscillation can not be available because a current injection region is large.
In application to an optical disc and the like, an output beam from a semiconductor laser is necessarily focused to a very small spot and therefore a fundamental transverse-mode oscillation is indispensable. However, in the InGaAlN based laser, a structure for a fundamental transverse mode stabilization has not been realized. In a conventional material system, for example an InGaAlP based system, only an SBR laser of a ridge stripe type has been reported in the following article:
3) M. Ishikawa et al., Extended Abstracts, 19th Conf. Solid State Devices and Materials, Tokyo (1987) pp. 115-118.
In an InGaAlN based laser, however, the structure used in the SBR laser cannot be applied without any change due to a difference in material system. As to a current confining structure in an InGaAlN based laser, a structure using GaN as a current confining layer is disclosed in the following application:
4) Jpn. Pat. Appln. KOKAI Publication No. 8-111558 (a semiconductor laser).
This structure works for current confinement but does not show a function of optical confinement and, thereby, there is difficulty producing an good quality output beam with small astigmatism and the like.
Generally, a composition, a thickness, a distance from an active layer and the like are necessarily set at respective designed values in order that a current confining layer formed in a cladding layer additionally works as a light confining layer. Especially in an InGaAlN based laser, even with the same composition, a completely different guiding mechanism is resulted in a laser according to a thickness and a position used because of a short wavelength. For this reason, an stable fundamental transverse mode oscillation is not provided only with incorporation of a current confining layer therein.
Also, if an Al containing layer is grown thick during a crystal growth process of an InGaAlN based materials, cracks are sometimes generated in the Al containing layer such as GaAlN layer, since there is a difference in lattice constant between an underlying GaN and the Al containing layer. For this reason, the transverse-mode confining in the direction of a layer (a vertical direction) does not work well so that a threshold value becomes extremately large or no guided mode is present.
On the other hand, a semiconductor laser used for an optical disc system, various specifications are specially required. Especially in write-once and rewritable types, a low power semiconductor laser for read and a high power semiconductor laser for erase/record are required and specifications for both are different from each other. Generally, a ultra thin film active layer is used for the high power semiconductor laser, but this structure is not necessarily suitable for a read laser. The reason why is that a low noise characteristic is required for the read laser and therefore, for example, a self-pulsation type structure is used, but the self-pulsation is hard to be obtained in the ultra thin active layer structure.
Under such circumstances, a high frequency superposition method or a combination of two kinds of laser has been adopted, but both are complicated in constitution. Besides, there has been reported a method, wherein two kinds of laser are formed using an active layer with variation of thickness according to positions, but in this method there arises a problem that controlling of thickness of the active layer is extremely difficult.
As described above, there have been proposed a variety of structures of and various methods of producing this kind of semiconductor laser, but no satisfactory characteristics have not been obtained in any cases, since crystal growth of a GaN based compound semiconductor layer is difficult. That is, the GaN based compound semiconductor layer cannot be grown as a good quality crystal and, therefore, carrier injection to an active layer cannot be effectively performed due to a poor crystal quality. In a structure wherein a stripe opening is formed in a current confining layer, a regrown layer after etching for formation of the stripe opening is degraded in crystal quality, which is an adverse factor for inviting a voltage drop in an electrode contact or the like.
To sum up, in order to realize a blue semiconductor laser with high reliability which works with a low threshold and a low voltage, and which is practically used for application of an optical disc and the like, effective current injection to an active layer and suppression of a voltage drop in a electrode contact or the like are important. However, in the current state of the art, a satisfactory structure with regard to the above points have not been obtained.
Moreover, semiconductor lasers of different wavelengths will be used more as an optical disc is progressed toward a higher density. In this trend, lasers of both wavelengths are sometimes in parallel needs, since interchangeability or compatibility is required between new and old optical systems. This situation will arise especially in the case where lasers have wavelengths of a large difference, such as in the case of a combination of red and blue in wavelength. The reason why is that a depth of a pit of an optical disc is optimized according to a wavelength of light source and, therefore, if a wavelength for read is greatly different, an SN ratio of a signal of reflection from a pit is reduced.
In such a way, in a conventional InGaAlN based semiconductor laser, a transverse-mode-stabilized structure is difficult to be produced and a laser which continuously oscillates in a fundamental transverse mode has difficulty being put to practical use.
For example, in an InGaAlN based semiconductor laser, it is difficult to produce a high-quality crystal layer because crystal growth thereof is difficult. A regrown layer after the etching for formation of a stripe opening has a further degraded crystallinity. For this reason, an efficiency in carrier injection to an active layer is reduced and, in addition, a voltage drop is caused by an electrode contact or the like. Accordingly, a device with high reliability which operates at a low threshold and a low voltage, and which is to be practically used for an optical disc and the like has difficulty being produced.
Moreover, degradation in crystallinity of a regrown layer on a GaN based compound semiconductor layer after the etching is held true to various semiconductor devices using a GaN based compound semiconductor.
Laser performances required for both cases of read and erase/record in an optical disc system are hard to be simultaneously realized.
There is difficulty in realization of a semiconductor laser which meet a need for compatibility between optical systems having different wavelengths of light sources and different recording densities.